Welcome to CS50 section! This is Week 5. Please open your CS50 IDE and run this in your console: cd ~/workspace/cs50-section ↵ git reset --hard ↵ git pull If new to this section, visiting, or want to “ start over ” , run this in your console: rm -r -f ~/workspace/cs50-section/ ↵ cd ~/workspace ↵ git clone https://github.com/bw/cs50-section.git The next pset is pset 5, after which you will have a take-home midterm. Afterwards, one week off to take a break (or participate in the coding contest!)
Know before attempting pset 5: Structs ● ○ Defining structs ○ Static v dynamic creation ○ Accessing fields ● Linked lists ● Hash tables ○ Tries ● Stacks and queues
Structs (aka structures) Encapsulate data of different types together ● ● Think “ object oriented programming ” typedef struct { char name[40]; char github[20]; int year; } student;
Structs (aka structures) What’s the datatype of this struct? ● typedef struct { char name[40]; char github[20]; int year; } student;
Structs (aka structures) What’s the datatype of this struct? ● struct student typedef struct { char name[40]; char github[20]; int year; } student;
Structs (aka structures) Created within the global scope ● ● To create a variable of type “ struct student ” : struct student brandon; ● To assign fields using the dot operator: strcpy(brandon.name, “Brandon Wang”); strcpy(brandon.github, “bw”); brandon.year = 2019;
Structs (aka structures) Before moving forward, be comfortable with-- ● Conceptual understanding of a struct ○ Good use cases for structs? ● How and where to define a struct How to add/modify fields of a struct ● ● How to modify string fields of a struct
Linked lists Before pset 5, review-- ● Creating a linked list ● Inserting into a linked list ○ At the head At the tail ○ ○ In the middle ● Deleting from a linked list ● Deleting an entire linked list Iterating over a linked list ●
Linked lists Conceptually, what’s a linked list?
Linked lists Conceptually, what’s a linked list? Programmatically, what’s a linked list?
Linked lists Conceptually, what’s a linked list? Programmatically, what’s a linked list? typedef struct node { // just some form of data; could be a char* or whatever int i; // pointer to next node; have to include `struct` since this is a recursive definition struct node *next; } node;
Linked lists Conceptually, what’s a linked list? Programmatically, what’s a linked list? typedef struct node { // just some form of data; could be a char* or whatever int i; // pointer to next node; have to include `struct` since this is a recursive definition struct node *next; } node;
Tricky things about linked lists How do we iterate over a linked list? ● ● How do we insert/delete a node? ○ Be careful of node orphaning
Tackling pset 5 Overarching decision you must make: What data structure do I use? ● Hash table ● Trie “ you should not encourage any student to pursue any implementation ”
Hash tables Associative array ● ● Position of each element determined by a “ hash function ” Hash functions: ● Take an input and generate a reproducible output Ideally: constant time output and few collisions ●
Hash tables Best of both worlds approach: Combines “ random access ability ” of an array with the “ dynamism ” of a linked list Assuming we define our hash table well: Insertions tend towards O(1) ● ● Deletions tend towards O(1) ● Lookups tend towards O(1)
Hash tables What defines a good hash function? Ideally-- ● Be deterministic ● Use only the data being hashed ● Use all of the data being hashed Uniformly distribute data ● For some hash function applications: ● Generate very different results for very similar (but different) data
Example hash function unsigned int hash(char* str) { intsum = 0; for (intj = 0; str[j] != ‘\0’; j++) { sum += str[j]; } return sum % HASH_MAX; }
Hash tables → Collisions Collisions occur when two pieces of data yield the same code. If storing data, we want both pieces of data. So we need to get both elements in the hash table.
Hash tables → Collisions Handling collisions ● ○ Linear probing ■ Clustering Chaining via linked lists ○
Tries
Tries Tries: roadmaps for hash tables. ● If you can follow the map from beginning to end, the data exists. ● If you can’t, it doesn’t exist. No collisions possible/allowed ●
Tries See CS50-standard slides for example scenario.
Stacks and queues We will talk about these conceptually today. Code snippets available after section (brandon.wang/cs50)
Stacks and queues We will talk about these conceptually today. Code snippets available after section (brandon.wang/cs50) What do stacks and queues do? ● Maintains data in an organized way Optimized for a specific type of data access ● ● Really bad for other forms of data access ● Usually we implement as an array or linked list
Stacks Stacks are last in, first out (LIFO). Allowed operations: ● Push ○ Add an element to the top of the stack Pop ● ○ Grab the most recently added element from the top.
Stacks typedef struct stack { VALUE array[CAPACITY]; int top; } stack;
Stacks typedef struct stack { VALUE array[CAPACITY]; int top; } stack; How would we push/pop elements?
Queues Queues are first in, first out (FIFO). Allowed operations: ● Enqueue ○ Add an element to the end of the queue Dequeue ● ○ Remove the (oldest) element from the front of the queue
Queues Best implemented as a linked list. typedef struct queue { VALUE val; struct queue *prev; struct queue *next; } queue; Maintain pointers to head AND tail of the list.
Data structures summary
Data structures Four primary ways we’ve looked at in CS50: ● Arrays ● Linked lists ● Hash tables Tries ●
Arrays Insertion ● ● Deletion ● Lookup ● Ease of sorting Size ●
Linked lists Insertion ● ● Deletion ● Lookup ● Ease of sorting Size ●
Hash tables Insertion ● ● Deletion ● Lookup ● Ease of sorting Size ●
Tries Insertion ● ● Deletion ● Lookup ● Ease of sorting Size ●
That’s all for today!
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